This circuit will trigger with any fluid with a resistance under 900K between the maximum separation distance of the probes. Let me explain further. The circuit uses a 4050B CMOS hex buffer wking on a 5 volt supply. All gates are biased off by the 10M resists connected between ground buffer input. The "common" probe the topmost probe above probe 1 in the diagram above is connected to the positive 5 volt supply. If probe 1 is spaced 1 cm away from the common probe tap water at 25 ?C is detected between the probes (a resistance of 20k) then the top gate is activated the LED 1 will light. Similarly if probe 2 at 2 cm distance from the common probe detects water, LED 2 will light so on. Switch 1 is used to whICh output from the hex buffer will trigger the audible oscillat made from the gates of a CMOS 4011B IC.Placement of Probes:As 7 wires are needed f the probe I reccommend the use of 8 way computer ribbon cable. The first two wires may be doubled act as the common probe wire. Each subsequent wire may be cut to required length, if required a couple of millimetres of insulation may be stripped back, though the open "cut off" wire end should be sufficient to act as the probe. The fluid distance between probe 6 the common probe wire must be less than 900k. This is because any voltage below 0.5 Volt is detected by the CMOS IC as logic 0. A quick potential check using a 900k resistance the divider fmed with the 10M resist at the input proves this point: 5 x (0.9 / (0.9+10) = 0.41 Volt.As this voltage is below 0.5 volt it is interpreted as a logic 0 the LED will light. If measuring tap water at 25 ?C then the distance between top probe common may be up to 45 cm apart. F other temperatures fluids, it is advisable to use an ohmmeter first. When placing the probes the common probe must be the lowest placed probe, as the water level rises, it will first pass probe 1, then 2 fin
